1. Field of the Invention
The present invention relates to a high frequency apparatus, specifically a high frequency apparatus including a uniplanar transmission line as a transmission line.
2. Description of the Related Art
In the coming 21st century, an advanced information and communication society fully equipped with information and communication infrastructure is expected to come. The demand for mobile communication terminals represented by cellular phones will be enhanced, and communication services having a higher speed and a larger capacity, for example, outdoor data communications services and moving picture communications services will be demanded. However, the frequency band currently used for cellular phones is not sufficiently wide for the high speed, large capacity communications. Therefore, a higher frequency band, i.e., a broader, millimeter wave band should be used.
When a higher frequency band is used, the wavelength of electromagnetic waves is shortened, and thus transmission lines used in a circuit are preferably shorter than the transmission lines in a conventional frequency band. When the transmission lines are unnecessarily long, the transmission loss is increased, resulting in deterioration in the performance of the circuit. Accordingly, when a higher frequency band is used, the size of the circuit is inevitably reduced. This requires conventional multi-chip ICs (MICs), including active elements and/or passive elements assembled on a substrate, to be replaced by monolithic microwave ICs (MMICs) including active elements and/or passive elements integrally produced on a substrate by semiconductor processing.
A GaAs substrate has a resistance of xcfx81=107 xcexa9cm, which is about 2000 times higher than that of an Si substrate. Therefore, a transmission line having a small transmission loss can be formed on the GaAs substrate, which is impossible with the Si substrate. This feature of the GaAs substrate, in combination with satisfactory high frequency characteristics of a GaAs-based device, is useful in realizing an MMIC.
Transmission lines can be roughly classified into a biplanar type and a uniplanar type. In the case of the biplanar transmission lines represented by microstrip transmission lines, a signal line is provided on a top surface of the substrate, and grounding lines are provided on a bottom surface of the substrate. Accordingly, when the structure of the circuit requires the signal line to be grounded, via-holes are needed for connecting the signal line on the top surface of the substrate to the grounding lines on the bottom surface of the substrate. Formation of the via-holes requires the substrate to be polished until the thickness of the substrate becomes a value of about 200 xcexcm to about150 xcexcm or less, which needs additional steps separate from the steps for producing the active elements. This reduces the yield and increases the cost, and thus is undesirable for practical use.
In the case of the uniplanar transmission lines represented by coplanar waveguides (hereinafter, referred to as xe2x80x9cCPWsxe2x80x9d), a signal line and grounding lines are formed on the same surface of the substrate. Accordingly, via-holes are not necessary, and thus the bottom surface of the substrate does not need to be polished. Therefore, the CPWs are advantageous for reducing the production cost of the MMICs.
The impedance of a CPW is determined by the distance between the signal line and each of the grounding lines (hereinafter, referred to as the xe2x80x9cline distancexe2x80x9d). Accordingly, impedance transform performed in order to match the impedance with the load is done by changing the line distance, for example, making a stepped portion in the CPW.
FIG. 6 is a schematic plan view illustrating an exemplary structure of a conventional CPW. FIG. 7A schematically shows ideal impedance transform.
In FIG. 6, a stepped portion is formed along line S to change the line distance in order to transform the characteristic impedance of the transmission line of Zo in an area to the left of line S into Zoxe2x80x2 in an area to the right of the line S (see FIG. 7A). However, when such a stepped portion is formed to change the line distance, parasitic impedance components (i.e., serial inductance component L and parallel capacitance component C) are generated in an area including the stepped portion and the vicinity thereof as shown in FIG. 7B. These parasitic impedance components cause an offset in the load impedance ZL, and as a result, the impedance of the CPW obtained by the impedance transform is offset from the load, without satisfactorily matching the load.
FIG. 7C is a Smith chart illustrating the impedance transform shown in FIG. 7B. It is assumed that a load impedane ZL is transformed through line 1 (FIG. 7A) having a characteristic impedance Zoxe2x80x2, using line 2 having a characteristic impedance Zo and a length of xcex/4 (xcex: wavelength of electromagnetic waves propagating through line 2). When line 1 is excessively short, ideally, impedance transform is performed along locus 1 (FIG. 7C). However, in actuality, the impedance ZL is offset by xcex94Z due to the influence of the parasitic impedance components of the stepped portion (serial inductance component L and parallel capacitance component C). Thus, impedance transform is performed from the point of ZL+xcex94Z along locus 2. As a result, the input impedance of the CPW with respect to the input side is Zinxe2x80x2 (FIG. 7B), not Zin (FIG. 7A) which is the intended value. Such an offset in the load impedance makes the circuit design difficult, especially in the high frequency range such as the millimeter wave band (30 GHz to 300 GHz).
When the impedance of a low impedance device such as, for example, a power FET (generally having an input impedance of, for example, about 6 xcexa9 or less) is to be transformed into 50 xcexa9 by a xcex/4 impedance transformer, the characteristic impedance of the xcex/4 transmission line should be 17 xcexa9 or less. However, a CPW provided on a GaAs substrate can have a line distance of about 5 xcexcm at the minimum, which provides a characteristic impedance of 30 xcexa9, due to the restriction by the thick film processing required by the plating method. Such a CPW is not preferable as an impedance transformer of a power device (i.e., low impedance device).
According to one aspect of the invention, a high frequency apparatus includes a dielectric substrate having a surface including a first area and at least one second area; a first dielectric thin layer provided on a portion of a first area; and a uniplanar transmission line provided on the first dielectric thin layer and on a portion of the second area, the uniplanar transmission line extending, continuously on the second area and the first dielectric thin layer.
In one embodiment of the invention, a dielectric constant of the uniplanar transmission line in the first area is different from a dielectric constant of the uniplanar transmission line in the second area.
In one embodiment of the invention, the surface of the dielectric substrate is exposed in the second area.
In one embodiment of the invention, the high frequency apparatus further includes a second dielectric thin layer provided on the second area of the surface of the dielectric substrate.
In one embodiment of the invention, a thickness of the first dielectric thin layer is larger than a thickness of the second dielectric thin layer.
In one embodiment of the invention, a thickness of the first dielectric thin layer is smaller than a thickness of the second dielectric thin layer.
In one embodiment of the invention, the first dielectric thin layer is formed of a dielectric material including an oxide of titanium.
In one embodiment of the invention, the second dielectric thin layer is formed of a dielectric material including an oxide of titanium.
In one embodiment of the invention, the first dielectric thin layer and the second dielectric thin layer are formed of a dielectric material including an oxide of titanium.
In one embodiment of the invention, the dielectric material including an oxide of titanium is SrTiO3.
In one embodiment of the invention, the dielectric material including an oxide of titanium is (Ba, Sr)TiO3.
In one embodiment of the invention, the first dielectric thin layer is formed of SiO1xe2x88x92xNx (0xe2x89xa6xxe2x89xa61).
In one embodiment of the invention, the second dielectric thin layer is formed of SiO1xe2x88x92xNx (0xe2x89xa6xxe2x89xa61).
In one embodiment of the invention, the first dielectric thin layer and the second dielectric thin layer are formed of SiO1xe2x88x92xNx (0xe2x89xa6xxe2x89xa61).
In one embodiment of the invention, the uniplanar transmission line includes a plurality of metal lines, and a line distance between the plurality of metal lines is changed in a stepped manner at a prescribed position.
In one embodiment of the invention, the line distance between the plurality of metal lines is changed in a stepped manner at an interface between the first area and the second area or the vicinity thereof.
In one embodiment of the invention, the uniplanar transmission line includes a plurality of metal lines, and a line distance between the plurality of metal lines is changed in a tapered manner at a prescribed position.
In one embodiment of the invention, the line distance between the plurality of metal lines is changed in a tapered manner at an interface between the first area and the second area or the vicinity thereof.
In one embodiment of the invention, the uniplanar transmission line is a coplanar waveguide.
In one embodiment of the invention, the dielectric substrate is a GaAs substrate.
In one embodiment of the invention, the dielectric substrate is a glass substrate.
In one embodiment of the invention, the high frequency apparatus further includes an active element on the GaAs substrate.
In one embodiment of the invention, the high frequency apparatus further includes an active element on the glass substrate.
According to another aspect of the invention, a high frequency apparatus includes a dielectric substrate; a uniplanar transmission line including a signal line and a pair of grounding lines having the signal line interposed therebetween; and a dielectric thin layer provided on a part of the dielectric substrate and below the signal line and at least a part of each of the pair of grounding lines.
Thus, the invention described herein makes possible the advantages of providing (1) a high frequency apparatus for appropriately matching the impedance with a load by suppressing the influence of parasitic impedance components, caused by a stepped portion or the like, on the load impedance so as to reduce the offset in the load impedance; and (2) a high frequency apparatus for transforming a low impedance of a load such as a power device or the like to an impedance of or around 50 xcexa9, which is the standard impedance, with easy and certainty.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.